Method for ignition of an oil burner and electronic ignition circuitry for oil burners

ABSTRACT

A method for ignition of an oil burner and electronic ignition circuitry for oil burners. Electronic ignition circuit for oil burners, for generating temporally spaced sparks. The electronic circuitry comprises circuitry for producing temporally spaced sparks with a spacing not corresponding to a half-period of the AC mains frequency or of a multiple of the AC mains frequency. The circuitry may be used in a method for ignition of an oil burner forming part of an oil burner system, where said oil burner comprises electronic circuitry for producing a first sequence of temporally spaced sparks for ignition of the oil. According to the method, the electronic circuitry is adjusted to subsequently produce a second sequence of temporally spaced sparks if it is detected that the ignition results in acoustic resonance in the oil burner system using said first sequence of temporally spaced sparks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/DK02/00269 filed on Apr. 25, 2002.

FIELD OF THE INVENTION

The present invention relates to a method for ignition of an oil burner forming part of an oil burner system, where said oil burner comprises electronic circuitry for producing a first sequence of temporally spaced sparks for ignition of the oil, as well as to electronic ignition circuitry for oil burners, for generating temporally spaced sparks.

BACKGROUND OF THE INVENTION

In the oil burners typically for heating in single-family houses, electric ignition is employed.

In such burners a blower creates an airflow into which the oil is sprayed through a spray nozzle to form an oil mist in the airflow. In order to ignite the oil an ignition spark gap is located in the vicinity of, but not too close to the spray nozzle. Typically the spark gap is located downstream from the blower and the spray nozzle with respect to the airflow. The distance from the spray nozzle to the spark gap in the direction of the airflow is, however, quite small, e.g. approximately 1-2 mm. The spray nozzle and the spark gap are usually off-set slightly in the direction across the airflow, so as to prevent the oil mist from reaching the electrodes of the spark gap. Other arrangements are of course possible. In particular, the spark gap could be located upstream from the spray nozzle.

On the other hand the spark gap must be located close enough to the spray nozzle to allow the arc formed in the spark gap to actually reach the oil mist and ignite it. When the arc is formed between the electrodes of the spark gap it will be deformed inter alia by the airflow so as to extend downstream into the oil mist.

As already stated, the present invention in particular relates to high voltage high frequency ignition. In FIG. 1 a known electronic circuit for producing the arc is shown. The circuit comprises a spark gap G connected to the secondary of a high voltage high frequency transformer T1. The electronic circuit incorporates an oscillator circuit R1, R2, R3, R4, C3, C4, C5, C6, DZ1, DZ2, TR1 and T1. It should be noticed that the transformer T1 is coupled with the basis of the transistor TR1, so as to provide the feedback needed for the oscillator.

The electronic circuit further comprises a half-wave rectifier circuit D1, C2 rectifying the 50 Hz AC mains supply, as well as noise suppression circuitry L1, C1, R5, R6, the details of which are not considered relevant for the present invention and will not be described in further detail. The 50 Hz AC mains is fed to the circuit as single phase AC on the terminals F and 0.

The oscillator is fed with the half-wave rectified current from the half-wave rectifier, and thus produces 50 high frequency bursts to the high frequency transformer T1 per second, whereby 50 sparks are generated in the gap G per second.

Though a prior art ignition unit with the above circuit has worked well over a number of years, it has been known that under certain circumstances acoustical resonance problems related to the ignition of the oil in the burner system occur. These problems have been increasing over recent years.

These acoustical problems occur in situations where the combustion has reached a self-sustaining state. The repeated occurrence of the ignition spark will influence the combustion, thereby creating flame fronts that will cause pressure pulses, rising and falling with 50 Hz, corresponding to the AC mains frequency.

If, in the above situation, it happens that these 50 Hz pressure pulses coincide with a resonance frequency of the system, e.g. of the Helmholz resonator formed mainly by the combustion chamber and the exhaust duct or of individual parts of the system, the system may be unacceptably noisy producing an annoying howling whenever ignition occur during combustion. In winter this may happen several times per hour, eg. 10 to 15 times.

Since the frequency at which resonance occurs often relate to the Helmholz resonator formed by the combustion chamber and the exhaust duct, the situations where this happens may not easily be predicted, as oil burner systems are often so as to include exhaust ducts in the form of existing chimneys. Thus, the noise problem is rarely discovered until the oil burner unit is fully installed. Moreover, if a new oil burner unit is bought, and installed with an existing exhaust duct, such as a brick chimney, it may be both complicated and costly to overcome the problem, as it involves detuning the system.

Also in recent years oil burner units have been reduced in size and weight. Those reductions have been accompanied with an increase in resonance problems on burner systems. Ways of avoiding and eliminating the problems are investigated intensely by the burner unit providers.

It is the object of the present invention to overcome this acoustic resonance problem in a simple, cost-efficient and versatile manner.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention this object is achieved by a method according to the opening paragraph, wherein the electronic circuitry is modified to subsequently produce a second sequence of temporally spaced sparks if it is detected that the ignition results in acoustic resonance in the oil burner system using said first sequence of temporally spaced sparks.

According to a second aspect of the present invention this object is achieved by electronic ignition circuitry according to the opening paragraph, wherein the electronic circuitry comprises circuitry for producing temporally spaced sparks with a spacing not corresponding to a half-period of the AC mains frequency or of a multiple of the AC mains frequency.

By the use of the method or the electronic ignition circuitry of the present invention, the frequency with which the ignition sparks occur in an oil burner may efficiently be de-tuned or de-correlated from the natural resonance frequency of the oil burner system, thereby preventing the unwanted resonance noise.

In a first preferred embodiment of the first aspect of the invention, the electronic circuitry is modified to produce the second sequence by suppressing some of the sparks in the first sequence.

Removing some of the sparks is an efficient way of preventing the build-up of resonance, which may moreover be realised in a cost-efficient way using the electronic ignition circuitry according to the second aspect of the invention.

In particular it has been found advantageous when the electronic circuitry is modified to suppress a unit fraction of the sparks in the first sequence, to suppress all but a unit fraction of the sparks in the first sequence, or to suppress a simple proper fraction of the sparks in the first sequence.

This is in particular advantageous as it allows the sparks to be omitted simply by gating the operation of the oscillator circuit supply using an on/off timer operating at a frequency different from the frequency of the pulsed supply current to the oscillator circuit.

In a preferred embodiment of the method modification comprises replacement of at least a part of said electronic circuitry.

That is to say if it is discovered upon installation that there is an acoustic problem, the engineer may be called and he will then replace the existing ignition unit by a unit according to the invention. This is advantageous in that it is a simple operation compared to modification of the exhaust duct or the burner unit itself.

In an alternative embodiment said modification comprises adjustment of the electronic circuitry.

This is advantageous if an embodiment of the ignition unit is used, in which a separate AC/AC or timer is used in connection with a prior art circuit, as described below.

In a different embodiment of the method according to the invention the electronic circuitry is adjusted to produce a second sequence of equidistant sparks, the sparks of said second sequence occurring at a different frequency than those of said first sequence.

This is advantageous as it allows continuous de-tuning of the ignition circuitry from the system. In particular this de-tuning may be realised by the control box of the oil burner, independently of the ignition circuitry as such.

According to an advantageous embodiment of the second aspect of the invention the electronic ignition circuitry comprises supply circuitry for repeatedly energizing said oscillator circuitry.

This allows simple generation of a series current supply pulses to the oscillator circuitry, which in turn generates a corresponding spark.

In the most preferred embodiment said supply circuitry comprises rectifying circuitry repeatedly supplying pulses of energizing current to said oscillator circuitry. Simple rectification of the typical 50 Hz AC mains then results in 50 pulses of supply current per second. Simple rectification, which results in the pulses mentioned, is preferred as it is a simple manner of achieving the sparks, which at the same time reduces the thermal load on the ignition circuitry, as compared to full rectification.

In further preferred embodiment, said circuitry for producing said temporally spaced sparks comprises circuitry for disabling the oscillator circuitry.

This allows for a simple omission of selected pulses, thereby creating a different temporal spacing between two pulses. In this respect it should be noted that the expression temporal spacing used throughout the description refers to the distance between corresponding parts of succeeding pulses, burst or sparks, as the case may be, i.e. between peaks, between the starting points of leading edges or between the ends of trailing edges.

In another preferred embodiment said circuitry for disabling the oscillator circuitry comprises circuitry for disabling the AC mains supply.

Disabling the mains supply constitutes a simple way of disabling the oscillator circuitry.

In another preferred embodiment said circuitry for disabling the oscillator circuitry comprises an on/off timing circuit. Use of an on/off timing circuit allows for generation of a gating signal which when superposed on the sequence of repeated supply pulses allows the omission of at least some of them.

In yet another preferred embodiment said circuitry for disabling the AC mains comprises control circuitry controlling the overall operations of an oil burner system.

This is advantageous in that the existing ignition unit may be employed when implementing the method according to the invention.

In a different preferred embodiment the electronic ignition circuitry comprises circuitry for frequency converting the AC mains frequency fed to the rectifier circuitry.

This is advantageous in that the spacing between the rectified supply pulses may be changed continuously by changing frequency of the input AC current.

The present invention further relates to a method for reducing the power dissipation in an electronic ignition circuitry for an oil burner, said electronic circuit being adapted for producing a first sequence of temporally spaced sparks.

According to the invention this may be achieved by a such a method, characterized in that depending on the thermal load, in particular the temperature, on the electronic ignition circuitry, the electronic ignition circuitry is adjusted to produce a second sequence of temporally spaced sparks differing from the first sequence.

In a preferred embodiment of the method said second sequence is produced by removing at least some of the sparks of the first sequence.

Removing at least some of the temporally spaced sparks will reduce the power dissipated.

In a further preferred embodiment of the method said sparks are generated by means of oscillator circuitry.

Using an oscillator circuit has the advantages stated above. Moreover, removing at least some of the temporally spaced sparks will reduce the power dissipated in the oscillator circuitry, as no substantial losses will occur in the oscillator circuitry, when it is not operated.

In a further preferred embodiment said sparks are removed by means of a temperature dependent timer disabling said oscillator circuitry.

This has the advantage that the temperature control may be integrated in the ignition circuitry. This again has the advantage that immediately as the circuitry heats up less power is dissipated. Thus, allowing a longer operation duty cycle of the electronic ignition circuitry compared to the prior art circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference to the drawings on which,

FIG. 1 shows a schematic diagram of a prior art ignition circuit,

FIG. 2 shows a schematic diagram of an embodiment of the ignition circuitry according to the invention,

FIG. 3 shows a block diagram of an alternative embodiment of the ignition circuitry according to the invention,

FIGS. 4-7 illustrate how different sequences of temporally spaced sparks are achieved using different timing sequences,

FIG. 8 is a schematic block diagram of an alternative embodiment of the ignition circuitry of the invention, and

FIG. 9 is a schematic diagram of a second alternative embodiment of the ignition circuitry of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram of a prior art electronic ignition circuit for oil burners.

The circuit comprises a spark gap G connected to the secondary of a high voltage high frequency transformer T1. The spark gap of the burner is located remote from the ignition unit containing the electronic circuit including the high voltage, high frequency transformer T1. The electronic ignition circuit incorporates an oscillator circuit R1, R2, R3, R4, C3, C4, C5, C6, DZ1, DZ2, TR1 and T1. It should be noticed that the transformer T1 is coupled with the basis of the transistor TR1, so as to provide the feedback needed for the oscillator.

The electronic circuit further comprises a simple half-wave rectifier circuit D1, C2 as well as noise suppression circuitry L1, C1, R5, R6, the details of which are not considered relevant for the present invention and will not be described further.

When the circuit is fed with AC supply current, the simple half-wave rectifier D1, C2 produces a series of distinct rectified pulses. These pulses supply the oscillator circuit R1, R2, R3, R4, C3, C4, C5, C6, DZ1, DZ2, TR1 and T1, which in turn produces a high frequency burst for each respective supply pulse. The envelope of the high frequency burst only to some extent follows the waveform of the supply pulses, but there will be a clear temporal correlation between them. Each of the bursts produce a corresponding ignition spark in the spark gap G.

This prior art circuit is inter alia advantageous in that it uses the commonly used AC mains supply frequency, i.e. that of the power grid, as a simple basis for generating temporally spaced ignition bursts. Since, in most countries the power grid is operated at 50 Hz, 50 equidistant pulses per second are produced by the rectifying circuit D1, C2, resulting in a corresponding number of equidistant bursts. If a different AC mains frequency, e.g. 60 Hz, is used, the number of bursts will differ correspondingly. In the following description it will however be assumed that supply frequency is 50 Hz, and that the acoustic resonance also occurs at 50 Hz.

Though it is generally advantageous to use the 50 Hz mains as a basis for the bursts, the use of the fixed mains frequency does in some cases lead to the acoustic problems mentioned above, which due to the fact that the frequency of the mains is fixed were difficult to overcome.

According to the present invention the resonance is broken, i.e. prevented from building up, by changing the spacing between at least some of the burst.

In a first embodiment, illustrated in FIG. 2, this is achieved by incorporating an on/off timer operating at a given duty cycle in the oscillator circuit.

The on/off timer is an astable timing circuit based on an integrated circuit commonly known as a 555 timer allowing a square signal with a given period to be output to the base of the transistor TR2. When the square signal is high the transistor TR2 conducts, thereby short circuiting the base of the transistor T1 to zero. Consequently the transistor TR1 is rendered non-conducting and thereby disables the oscillator circuit.

The on/off timer is supplied via a voltage divider R7, R8 with a smoothing capacitor C7 ensuring the supply of the timer between the voltage pulses from D1.

The 555 timer has two comparator inputs 6 and 7. During the time when the 555 timer is on the capacitor C8 is charged through the resistors R9 and R10. When the voltage difference between the inputs 6 and 7 falls below a predetermined threshold because the capacitor C8 is charged the output 3 of the 555 timer will switch to off.

When the 555 timer switches to off, the capacitor C8 discharges through the resistor R10 and the comparator input 7.

Thus the on timing periods of the timing circuit is determined by the resistors R9 and R10 and the capacitor C8 and the off timing periods by the resistor R10 and the capacitor C8.

R12 serves to limit the current drawn from the output 3 of the 555 timer.

FIGS. 4 a, 5 a and 6 a illustrate with a broken line the enabling signal on the basis of TR1 for various given periods for the square signal together with the envelope of the bursts as they would be generated if the on/off timer was not present. It should be noted that the output 3 of the 555 timer is the inverse of the enabling signal for the oscillator circuit in FIGS. 4 a, 5 a and 6 a, as high potential on the base of the transistor TR2 makes the transistor TR2 conduct, which disables the oscillator circuit.

FIGS. 4 b, 5 b and 6 b, illustrate the envelope of the bursts produced by the oscillator circuit when the enabling signal is applied to the oscillator circuit. In these figures as well as in FIG. 7 the envelope is illustrated in arbitrary voltage units as a function of time in milliseconds. The abbreviation std. EBI refers to the bursts as they would have been produced using the standard EBI type prior art device manufactured by the applicant.

In FIG. 4 a the square signal has a 20 ms on time followed by a 20 ms off time. The square signal being the enabling signal for the oscillator circuit is timed to rise and fall together with the rise of the envelope of the bursts, i.e. not during the bursts.

As can be seen from FIG. 4 b, the timing period of 20/20 ms enables every other of the bursts, which would otherwise have been produced by the oscillator circuit. It has been found that this halving repetition rate of bursts, and thus the sparks, to 25 Hz is in many cases sufficient to overcome the resonance problems.

In other words the square timing signal removes a unit fraction of the bursts, a unit fraction being a fraction 1/N where N>1. N=2 in the case illustrated. In particular, a simple unit fraction of the bursts is removed. A simple unit fraction being a unit fraction, where N is an integer as opposed to e, n or {square root}N etc.

In FIG. 5 a the square signal has a 30 ms on time followed by a 30 ms off time. The square signal is again timed to not rise or fall during a burst.

As can be seen from FIG. 5 b, the timing period of 30/30 ms disables every third of the bursts, which would otherwise have been produced by the oscillator circuit. It has been found that removing some of the bursts in this way, efficiently overcomes the resonance problems. Also in this case a unit fraction of the bursts are removed, viz. ⅓ of the bursts.

In FIG. 6 a the square signal has a 30 ms off time followed by a 20 ms on time resulting in the burst sequence of FIG. 6 b, which also efficiently overcomes the acoustic resonance problems.

This embodiment does not remove a unit fraction of the bursts but a simple proper fraction of them. A simple proper fraction being a fraction M/N, where both M and N are integers and N>M.

The ignition circuit of FIG. 2 will generally constitute a self-contained moulded-in unit with the on/off timer set for any one of the above timing periods. In the case illustrated ⅗ of the sparks are removed.

If, upon installation of the burner, which will typically include the prior art circuit, acoustic resonance is detected, the engineer may be called and the problem be overcome by having him installing the new and inventive ignition unit instead of the prior art unit.

It should be noted that the above on/off timing periods, though presently preferred, are merely illustrating examples, and that other timing periods, e.g. removing more bursts may equally be used.

According to a second embodiment of the invention, illustrated in FIG. 3, the above resulting burst sequences may also be realised by means of electronic ignition circuitry comprising a combination of an on/off timer and the prior art circuit of FIG. 1. The timer is arranged so as to enable or disable the supply to the prior art ignition circuit. By interrupting the supply appropriately, some of the current supply pulses to the oscillator will be omitted and corresponding burst consequently not produced.

The timer could be replaced by a control box including a control circuit, which controls the overall operation of the burner system, where the control circuit enables and disables the supply to the prior art ignition circuit.

This has the advantage over the unit described above, that the prior art unit may still be used, and only supplemented with the timing circuit. The timing circuit may then have the same fixed values as described above, or it may be externally controllable to be switched to the most appropriate among them for a given acoustic problem.

As mentioned above the use of supply pulses for the oscillator is advantageous in the sense that it reduces the thermal load on the ignition circuitry. The prior art circuit of FIG. 1 is designed for a duty cycle of ⅓ of three minutes. That is to say the ignition circuitry may operate up to one minute of every three minutes.

By omitting some of the pulses, this thermal load from heat dissipated in the ignition circuit when the bursts are produced, is reduced. This has the advantage that the output power of each burst may be increased, for the same duty cycle of ⅓. This contributes to early ignition of the oil mist and thus reduces pollution. I.e. under ideal circumstances when the burner is well adjusted, the oil mist should ignite already upon the first ignition spark. This is however not always so, but with the increased power of the sparks, the ignition sparks are provided with more power and thus increase the likelihood of an ignition of the oil mist upon one of the first sparks, and therefore contribute to early ignition even if the burner is not optimally adjusted.

In a special embodiment of the invention, illustrated in FIG. 9, this feature may be utilized to increase the duty cycle of the ignition circuit.

This is achieved by controlling the timing circuit by means of a temperature sensor.

In a preferred variant of this embodiment where the power of the sparks corresponds to a standard EBI, the temperature sensor is a PTC resistor, i.e. a resistor with a positive temperature coefficient, arranged in parallel with R9 within the ignition unit. Using a PTC resistor R11 in parallel with R9 will make the on/off timing dependent on the temperature in the ignition unit, in such a way that the duration of the on time of the oscillator circuit is kept constant while the off period extends with increased temperature.

If the on/off timing of the enabling signal for the oscillator is 30 ms on and 30 ms off under normal temperature the ignition unit will it-self disable some of the bursts when the temperature increases, thereby reducing the heat dissipated in the unit. This makes the unit more resistant to overheating, and it is thus possible to increase the duty cycle to more than ⅓ as was the maximum for the prior art unit.

It should be noted that because the PTC changes its resistance value continuously some of the bursts may not correspond to a half period of the AC mains frequency, but only to a part thereof as the timing circuit may enable or disable the oscillator in the middle of such a period.

Thus, ideally a sequence of bursts according to FIGS. 7 a and 7 b may be achieved. It, should however be noticed that for purposes of illustration the rate of change in the period of the on/off timing cycle is somewhat exaggerated in comparison to what could be expected in practice. In practice the rate of change will be slower due to the fact that the unit is moulded into a mass of plastic material. The total mass of the unit will therefore slow down the overall temperature change of the unit. In FIG. 7 a the sequence of 50 Hz bursts are shown together with the enabling signal for the oscillator circuit, where the on and off periods of the enabling signal change gradually over time.

As can be seen, the on/off switch of the timing circuit is not in this embodiment synchronised with the rise of fall of the bursts. Thus some short sparks are produced. Even if those short bursts are not fully omitted they have a substantially reduced power, and will thus not dissipate so much heat in the circuitry as would the production of a full burst.

Though the above embodiments in which bursts are removed from the 50 Hz sequence in order to break resonance, this may also be achieved with different means. Thus, in a different embodiment of the invention shown in FIG. 8, the temporal spacing is modified by changing the frequency of the AC supply current fed to the circuit on the phase F and zero 0 by means of an AC/AC converter. Using the AC/AC converter is less cost efficient that using the above timing circuits.

Finally it should be noted that the invention is not limited to the described embodiments and configurations, in particular the mains frequency may be any commonly used mains frequency such as 50 or 60 Hz. Moreover it will be evident to the skilled person that a variety of other timing circuits than the astable 555 based timing could be used. It should also be noted that even though the described embodiment relates to the use of an ignition circuit using a high frequency oscillator, the invention may also be applied in systems where a high voltage transformer transforming the AC mains directly, i.e. without the use of an oscillator circuit. 

1-21. (canceled)
 22. A method for ignition of an oil burner forming part of an oil burner system, where said oil burner comprises electronic circuitry for producing a first sequence of temporally spaced sparks for ignition of the oil, causing the electronic circuitry to produce a second sequence of temporally spaced sparks if it is detected that the ignition results in acoustic resonance in the oil burner system using said first sequence of temporally spaced sparks.
 23. A method according to claim 22, wherein the electronic circuitry is modified to produce the second sequence by suppressing some of the sparks in the first sequence.
 24. A method according to claim 23, wherein the electronic circuitry is modified to suppress a unit fraction of the sparks in the first sequence.
 25. A method according to claim 23, wherein the electronic circuitry is modified to suppress all but a unit fraction of the sparks in the first sequence.
 26. A method according to claim 23, wherein the electronic circuitry is modified to suppress a simple proper fraction of the sparks in the first sequence.
 27. A method according to claim 22, wherein said modification comprises replacement of at least a part of said electronic circuitry.
 28. A method according to claim 22 wherein said modification comprises adjustment of the electronic circuitry.
 29. A method according to claim 22, wherein the electronic circuitry is adjusted to produce a second sequence of equidistant sparks, the sparks of said second sequence occurring at a different frequency than those of said first sequence.
 30. An electronic ignition circuitry for oil burners, for generating temporally spaced sparks, the electronic circuitry comprising; circuitry for producing temporally spaced sparks with a spacing not corresponding to a half-period of an AC mains frequency or of a multiple of the AC mains frequency.
 31. An electronic ignition circuitry according to claim 30, wherein the electronic ignition circuitry comprises an oscillator circuit for generating high voltage high frequency bursts.
 32. An electronic ignition circuitry according to claim 31, further comprising supply circuitry for repeatedly energizing said oscillator circuitry.
 33. An electronic ignition circuitry according to claim 32, wherein said supply circuitry comprises rectifying circuitry repeatedly supplying pulses of energizing current to said oscillator circuitry.
 34. An electronic ignition circuitry according to claim 31, wherein said circuitry for producing said temporally spaced high frequency bursts comprises circuitry for disabling the oscillator circuitry.
 35. An electronic ignition circuitry according to claim 34, wherein said circuitry for disabling the oscillator circuitry comprises circuitry for disabling the AC mains supply.
 36. An electronic ignition circuitry according to claim 34, wherein said circuitry for disabling the oscillator circuitry comprises an on/off timing circuit.
 37. An electronic ignition circuitry according to claim 35, wherein said circuitry for disabling the AC mains comprises control circuitry controlling the overall operations of an oil burner system.
 38. An electronic ignition circuitry according to claim 33, further comprising circuitry for frequency converting the AC mains frequency fed to the rectifier circuitry.
 39. A method for reducing the power dissipation in an electronic ignition circuit for an oil burner, comprising the steps of: causing said electronic circuit to produce a first sequence of temporally spaced sparks for the ignition of the oil, and depending on the temperature of the electronic ignition circuit, adjusting the electronic ignition circuit to produce a second sequence of temporally spaced sparks differing from the first sequence
 40. A method according to claim 39, wherein said second sequence is produced by removing at least some of the sparks of the first sequence.
 41. A Method according to claim 39, wherein said sparks are generated by means of oscillator circuitry.
 42. A method according to claim 41, wherein said sparks are removed by means of a temperature dependent timer disabling said oscillator circuitry. 